Mica sheet and process for making the same



- Jan. 9, 1945. p G. PELLETT 2,37,047

MICA SHEET AND PROCESS FOR MAKING THE SAME Filed Feb,. 18, 1941 POLY METHYL M E THA CR YL ATE BONDED MICA SHEET Inventor: Fred G. Pelle b,

b 6.. W4 y His Attorhe'y Patented Jan. 9, 1945 MICA SHEET AND PR THE ocE'ss FOR MAKING SAME Fred G. Pellett, Alplaus, N. Y., assignor to General Electric Company, a corporation of New York Application February s, 1941, Serial No. 379,541 M 2 Claims.

This invention relates to articles comprising bonded mica flakes or fragments. It is particularly concerned with the surfacing of such ar-- ticles with thin, smoothocoatings of a thermoplastic resin such as a methacrylate resin.

Because natural mica cannot be obtained readily in sheets of suitable area and thickness, electhe maximum possible amount of mica per unit volume of insulating material is desired. Furthermore, it has been found that in punching mica sheets surfaced with chemically treated cellulosic sheet materials there is some tendency for the cellulose layers to separate from the un- -derlying mica layer with the production of trically insulating mica sheets ordinarily comprise comparatively small mica flakes bonded together by means of a suitable binder. Copal resins, alkyd resins and, in particular, shellac have been' widely used as organic binders therefor. In accordance with the usual practice, the individual mica flakes are coated with a thin film of the' binder, for example, from a solution thereof, are then dried to removeathe solvent and are thereafter assembled in 511011 a manner that upon pressing between hot parallel press plates an in-, sulating sheet'of considerable area is obtained. Due to the shape of the individual mica fragments, very little flowing or slippage thereof takes place during the pressing operation so that with thetprocess generally used for arranging .and coating the flakes prior to pressin pressed sheets of non-uniform or uneven thicknessare ordinarily obtained. As most electrical applications require insulating sheets of a more uniform thickness,

frayed or delaminated' edges obviously as undesirable in the final product as are the feathery surfaces of the uncoated sanded sheets.

An object of the present invention is to overcome and eliminate the above defects by providing mica sheets having thin flrmly-adhen ing surface coatings of a non-sticking material which will not materially increase the total organic content of the sheets..

the surface of these pressed mica sheets are 7 usually sanded or otherwise surface finished to obtain a product of uniform thickness. This surface-finishing of the sheet results in the exposure of mica flakes which are no longer coated with the binder and which tend to flake off or feather. Such sheets or segments cannot be used, for example, in the assembly of commutators by means of automatic machines. Their sanded, surfaces are not particularly slippery and in that the resultant surfaces are not very smooth or slipp ry nor are they entirely free from a tendency towards flaking of the mica. It has also been proposed to surface bonded mica sheetswith thin sheets of chemically treated cellulose suchascellulose acetate. However, the thinnest sheets of such material commercially available produce a marked over-all increase in the organic content of the mica insulating material which is obviously undesirable where for electrical. reasons Another object of the invention is to provide the methacrylate resins, i.' e., the J olymerized esters of methacrylic acid such as polymethylmethacrylate, were the only onesfound capable of meeting all of the above-mentioned objects.

In accordance with my invention, the mica flakes or fragments which have previously been coated with a suitable binder such as' shellac are,

thereafter assembled and hot pressed into the form of alaminated mica sheet in accordance,

with the well known practice. The pressed sheet is sanded to the desired thickness and the sanded sheets are coated with a thin film of methacrylate resin as from a suitable solution thereof. An example of a suitable coating composition is one of the foliowing composition: Per cent Methyl methacrylate resin 8 Montan w i 0.75 Toluol- 91.25

The mica sheets may be coated with, this solution 9 using ordinary coating rollers; l-lowever, any

other convenient method may be used such as brushing or dipping. After the solvent has evaporated the coated sheets are hot pressed between smooth steel press plates to obtain the finished product.

1 The montan wax called for in the above for mula acts as a lubricant during the pressing of the coated sheets. It is to be understood that other lubricants may be used orthe lubricant may be omitted from the {coating formula and a plied instead directly to the pressplates. Solvents other than toluol may be'uscd andthe concentration of the solution may be varied to suit the coating method or to obtain the desired film thickness.

Preferably in order to keep the organic content of the finished sheets as low as possible, the coating solution should be rather dilute. For practical applications, the concentration thereof should not exceed approximate1y20 per cent. The calculated film thickness of the surface film obtained with the above-mentioned 8 per cent methacrylate composition averages about 0.00009 inch. With a 30 mil mica sheet containing approximately per cent binder, which is the average binder content of bonded mica segments for smallcomxnutators, the application of such thin films of the methyl methacrylate resin adds about 0.25 per cent organic material to the-sheet representing approximately a 5 per cent increase in total contentof organic material. Preferably,

the surface films of methacrylate resin should not exceed substantially 0.0001 inch in average thickness. An outstanding advantage of the methacrylate surface coatings results from the fact that the sheets coated therewith may be hot pressed without sticking to the'heated pre'ss plates. Hot

edges of the sheets or segments during p nching and handling when surfaced with the met-ha crylate resins.

Although the surfaced sheets of my invention are suitable for use wherever laminated mica sheets have previously been used, they are particularly adapted, as has previously been stated, in the machine assembly of small commutators where it is essential that the small segments readily slide one over the other without the mica edges thereof tending to catch 'on'the surfaces of the adjacent mica segments or on partsof the machine. There is no tendency for the methacrylate coated sheets to adhere to one another in the machine under ordinary summer heat and pressure conditions.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An article of manufacture comprising a composite sheet of shellac-bonded mica flakes sanded to uniform thickness, the said sanded sheet having a thin, smooth surface coating of a methacrylate resin not exceeding substantially 0.0001 inch in average thickness intimately bonded thereto.

, 2. The-process 01 making a composite mica sheet for use in the machirie assembly of commutator segment insulation which comprises hot pressing produces a much smoother surface than ness, applying a dilute solution of a methacrylate resin to the surfaces of said sanded sheet, permitting the solvent to evaporate and again hot pressing the coated sheet.

FRED G. PELLE'I'I. 

